Fix form submission's replace-before-load logic

[domenic]

In 524485a, we overhauled the logic which converts "push" navigations to "replace" navigations before the load event fires. However, for form submissions we erroneously checked the loading state of the form's document, instead of the loading state of the target document; they can be different when using the form's target="" attribute.

Further investigation reveals that this area is not interoperable, and neither Gecko nor WebKit pass tests for the "push"-to-"replace" conversion for such cross-document form submissions. (It's not clear exactly what they do, as they don't pass tests for the opposite behavior either.) Since we don't have interop here, we can standardize on a simpler behavior, of just avoiding the "push"-to-"replace" conversion.

Closes #9135.

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• At least two implementers are interested (and none opposed):
  • This area is generally not interoperable enough, so right now nobody passes the tests below. But both Gecko and WebKit implement something closer to this, than to the existing spec.
  • Gecko is willing to implement this
  • Chromium is willing to implement this
• Tests are written and can be reviewed and commented upon at:
  • Add more tests for <form>.submit() and replace-before-load web-platform-tests/wpt#39516
• Implementation bugs are filed:
  • Chromium: https://bugs.chromium.org/p/chromium/issues/detail?id=1440574
  • Gecko: N/A
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=256069
  • Deno (only for timers, structured clone, base64 utils, channel messaging, module resolution, web workers, and web storage): N/A
  • Node.js (only for timers, structured clone, base64 utils, channel messaging, and module resolution): N/A
• MDN issue is filed: N/A, too small a change

(See WHATWG Working Mode: Changes for more details.)

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/form-control-infrastructure.html ( diff )

[annevk]

Is this for all form submissions or only POST? It would make more sense to me if that was the dividing line and <form method=get> was the same as <a>.

[domenic]

This is for all form submissions. I think form method=get is more similar to form method=post, than it is to anchor elements. (And in terms of both spec and implementation code, that seems to be the case, in terms of requiring fewer special cases.)

[annevk]

I guess I'll rubberstamp, but it doesn't make a whole lot of sense to me that form navigations have different session history behavior from other navigations. I could see that for POST navigations though.